Rust inhibitive and paint holding treatment for alloyed zinc iron surfaces



United States Patent RUST INHIBITIVE AND PAINT HOLDING TREAT- MENT FOR ALLOYED ZINC IRON SURFACES George R. Hoover, Middletown, Ohio, assignor t0 Armco Steel Corporation, Middletown, Ohio, :1 corporation of Ohio No Drawing. Filed Mar. 22, 1960, Ser. No. 16,657

3 Claims. (Cl. 1486.16)

This invention resides in a process and solution for treating alloyed zinc coatings, that is, zinc-iron and/or iron-zinc alloyed surfaces, to prevent white rust corrosion (storage stain) under mildly corrosive conditions, and to provide suitable substrate for organic coatings such as paint, lacquer and enamels.

Some time ago the old fashioned galvanized sheet material was developed. This consisted of a layer of zinc metal, with some iron-zinc alloy at the interface, bonded onto steel. Two difliculties with this type material were soon experienced. One, the zinc surface reacted with paints, lacquers and most decorative and protective organic coatings, causing said coatings to become brittle and to lose their adherences to the surface. Two, during storage a white rust would form. This white rust, while not having an undue influence on the total corrosion resistance and ultimate life of the material, does make the material unsightly and is quite undesirable. It was later discovered that the paintability of this old fashioned galvanized sheet material could be improved by subjecting the sheet to a particular, relatively short, heat treatment as disclosed in United States Patent 2,407,881. This process, however, did not eliminate the white rust problem.

Another similar type of material was developed and became known as Zincgrip. In this material the zinc coating metal has a small percentage of aluminum in it which prevents much alloy formation between the zinc and steel. In United States Patent 2,487,137 it was taught that this Zincgrip could be made paintable by treating the material with certain phosphate solutions containing fluorine but without actually producing coatings which contained fluorine in anything but very small amounts. Zincgrip has the typical galvanize coating spangle, and upon aging it was found that phosphate coated Zincgrip would corrode at the spangle boundaries and cause organic coatings to blister in this area.

An attempt was made to prevent white rusting of galvanized and aluminum coated ferrous materials such as Zincgrip. United States Patent 2,784,122 taught that white rusting of Zincgrip could be prevented by treating the surface of the galvanized or zinc-aluminum coated material with a solution of chromic acid in pure water under certain conditions. This solution, however, by itself, did not make the treated material more paintable.

In addition to the old or ordinary galvanized sheet material and to the Zincgrip sheet material above discussed, there was developed an alloyed zinc coating (with aluminum) which was heat treated so as to produce from a Zincgrip type of alloy, 2. final product in which all the coating metal is in the single phase alloy stage. In this third type of sheet material, the alloyed zinc coating, after heat treating, is substantially homogeneous and is made up of zinc plus about 8 to 12% iron and about .13% aluminum.

Since the chromic acid solution treatment of Patent 2,784,122 had worked on Zincgrip to prevent white rust, it was suggested that perhaps this same chromic acid process would work on the alloyed zinc coating for the same purpose. This was tried and it was found that it did not prevent white rust in the alloyed zinc coatings, although, surprisingly enough, it did have a beneficial 3,074,827 Patented Jan. 22, 1963 "iceeffect on the paintability of these alloyed zinc coatings whereas it had no such effect when used on the Zincgrip. The problem, therefore, was to find a process and solution for treating alloyed zinc coatings so as to prevent white rust and at the same time improve paintability. From the above it will be readily apparent that thesezinc-iron and/or iron-zinc alloyed surfaces presented a white rust and paintability problem which was not solved by applying what had heretofore been known in the art; thus the instant invention.

It has now been found, and this is the subject invention, that white rust in alloyed zinc coatings containing aluminum may be prevented, and the paintability of the coatings improved, by treating the substantially singlephase, alloyed surfaces with a water solution of chromium trioxide (cromic acid) and ortho-phosphoric acid.

Before continuing with a detailed description of the invention, certain observations are pertinent. It has already been pointed out that the heretofore known solution of chromic acid in pure water, effective to prevent white rusting of Zincgrip, is inefiective to prevent white rusting of the alloyed zinc coating, i.e., the substantially single phase, alloyed coating made up of zinc plus about 8-12% iron and about .13% aluminum. Conversely, the solution of this invention (a water solution of chromic acid and phosphoric acid), applied to Zincgrip rather than to the alloyed zinc coating, detracts from white rust prevention and does not improve paintability. The lack of interchangeability of these various solutions is quite surprising and made the solution to the problem at hand difiicult. By way of further example, as also briefly noted hereinbefore, the previously known solution of chromic acid in pure water, when applied to the Zincgrip for which it was intended as a white rust preventer, had no beneficial effect on the paintability of the Zincgrip. Yet this old solution, while not effective to prevent white rusting of the alloyed zinc coating, was effective to improve the paintability of such alloyed zinc coatings.

The treating solution of the instant invention is applied to the alloyed surface at temperatures below the boiling point of water. The solution is dried on the treated surface by any convenient means at any temperature below 325 Fahrenheit. The temperature of application of the solution may be varied from room temperature up to the boing point of water and the optimum temperature is about Fahrenheit. Room temperature, however, will suffice, but if the solution is hotter, the solution will dry itself rather than necessitating the use of hot air blowers and the like to dry the solution. The reason for drying below 325 Fahrenheit is to prevent the loss of some of the chrome which is believed would occur if the temperature were above this. It is thought that the hexavalent chromium will go over to trivalent chromium if the temperatures go very much above this range.

The solution may be applied straight off of the zinc coating line or after or during the temper rolling of the zinc coated material. Temper rolling accomplishes a slight reduction in thickness of the coated material, and this reduction causes heating up of the strip. Thus, when this phosphoric acid-chromic acid solution is applied in or during the temper rolling step, the solution itself does not have to be heated above room temperature because the heat in the strip will cause a rapid drying. This enables the treating solution to be applied to the temper mill as the coolant solution doing the double job of cooling the mill so that excessive temperatures are not developed and at the same time being present in the quantities necessary to do an adequate job of coating the alloyed zinc coated sheet.

Equal parts of phosphoric acid and chromic acid have produced satisfactory results but the preferred range is 2 parts phosphoric acid and 1 part chromic acid. The preferred concentration of acid is 1% phosphoric acid and /t chromic acid, by weight. With respect to these percentages of the acids in the water solution, it is preferred that they do not go above substantially 10% phosphoric acid and chromic acid. Also, as a low limit, there should be about /2% phosphoric acid and fit% chromic acid.

The solution may also be applied to the alloyed zinc coating surface to be treated by spray, brush immersion, or by coating rolls. The solution of phosphoric acid and chromic acid reacts with the alloyed surface to produce a mixture of phosphates and chromates. The phosphates, determined as zinc phosphate, ZN (PO should be in the range of .028 gram per square foot of sheet to .200 gram per square foot of sheet. The chromium content of the film (determined as chromium) should be in the range .0005 gram to .004 gram per square foot of sheet (2 square feet of surface). It is possible that a solution more dilute than that above indicated might be used so long as the residue (film obtained) is within the ranges just set forth.

It has been found that the ratio of 1 part chromic acid to 2 parts phosphoric acid works well. This ratio may even be more important than the actual amounts used, assuming, of course, that the film' obtained is in the specified ranges. As indicated, however, equal parts have been used. When, however, the percentage of chromic acid gets too high, the phosphoric acid reaction will slow down. Apparently the use of too much chromic acid produces an oxidizing effect which inhibits the effectiveness of the phosphoric acid. The amount of phosphate deposited is preferably about three times that of the chromate. And, if the percentage of phosphoric acid gets too high, it will tend to pickle rather than to carry out the chemical reduction of chromium on the sheet. The final result of the reaction of the phosphoric acid and the chromic acid on the alloyed zinc coated surface is to produce a Zn (PO -CrPO complex.

It is believed that the invention has been fully described above. Briefly summerized, it resides in treating a steel strip to which a zinc coating (with aluminum) has been alloyed with a water solution of phosphoric acid and chromic acid with a preferred concentration in the solution of 1% phosphoric acid and /z% chromium trioxide, by weight. The solution is applied in the neigh- 4 borhood of Fahrenheit, in any event under the boiling point of water, and is dried on the treated surface at temperatures below 325 Fahrenheit.

Although the invention has been described in terms of preferred ranges and conditions, the invention is not to be limited to these particular ranges and conditions except insofar as they are set forth in the subjoined claims. Variances are permissible as indicated and modifications are Within the scope and spirit of the invention.

Having thus described the invention, what is claimed as new and what is desired to be protected by United States Letters Patent is:

l. A process of treating a sheet having a substantially single phase, alloyed zinc coating containingaluminum to prevent white rustand improve paintability which comprises the steps of: preparing a water solution of phosphoric acid and chromic acid in which the" concentration of acid is /2 to 10% phosphoric acid and to 5% chromic acid, the percentages being" by weight, and in which the quantity of phosphoric acid is at least substantially as great as the quantity of chromic acid; applying the solution to the zinc coated sheet at a temperature below the boiling point of water and in sufficient quantity to form phosphates in the range of .028 gram to .200 gram per square foot of sheet, and to form chromates on the coating in the range of .0005 gram to .004 gram of chromium per square foot of sheet; and drying the solution on the coating at a temperature below 325 Fahrenheit. 7

2. The process of claim 1 in which the solution is applied to the coating at about 160 Fahrenheit.

3. A steel product having an alloyed zinc coating with aluminum, in which the coating metal is in the single phase alloy stage, having on the coating a zinc phosphate-chromic phosphate complex, said complex including phosphates in the range of .028 gram to .200 gram per square foot of coating, and said complex also including chromates in the range of .0005 gram to .004 gram per square foot of coating.

References Cited in the file of this patent UNITED STATES PATENTS 2,163,583 Buzzard June 27, 1939 2,200,615 Boyle May 14, 1940 2,303,242 Tanner et a1 Nov. 24, 1942 2,412,543 Tanner Dec. 10, 1946 

1. A PROCESS OF TREATING A SHEET HAVING A SUBSTANTIALLY SINGLE PHASE, ALLOYED ZINC COATING CONTAINING ALUMINUM TO PREVENT WHITE RUST AND IMPROVE PAINTABILITY WHICH COMPRISES THE STEPS OF: PREPARING A WATER SOLUTION OF PHOSPHORIC ACID AND CHROMIC ACID IN WHICH THE CONCENTATION OF ACID IS 1/2% TO 10% PHOSPHORIC ACID AND 1/4% TO 5% CHROMIC ACID, THE PERCENTAGES BEING BY WEIGHT, AND IN WHICH THE QUANTITY OF PHOSPHORIC ACID IS AT LEAST SUBSTANTIALLY AS GREAT AS THE QUANTITY OF CHROMIC ACID; APPLYING THE SOLUTION TO THE ZINC COATED SHEET AT A TEMPERATURE BELOW THE BOILING POINT OF WATER AND IN SUFFICIENT QUANTITY TO FORM PHOSPHATES IN THE RANGE OF .028 GRAM TO .200 GRAM PER SQUARE FOOT OF SHEET, AND TO FORM CHROMATES ON THE COATING IN THE RANGE OF .0005 GRAM TO .004 GRAM OF CHROMIUM PER SQUARE FOOT OF SHEET; AND DYING THE SOLUTION ON THE COATING AT A TEMPRATURE BELOW 325* FAHRENHEIT. 